Apparatus for production of chromic acid and caustic alkali



Sept. 29, 1936. J. Waoss APPARATUS FOR PRODUCTION OF CHROMIC ACID ANDCAUSTIC ALKALI Original Filed Feb. 16, 1935 flu lhwentor GttomegaPatented Se t. 29,- 1936 APPARATUS FOR PRODUCTION OF CHRO- IVHC ACID ANDCAUSTIC ALKALI John W. Boss,Livingston, Mont. Original applicationFebruary 16, 1935, Serial No.

6,904. Divided and 1935, Serial No. 25,653

this application June 8,

Claims. (Cl. 204-5) The present application is a division of mycopending application No. 6,904 filed February 16, 1935.

containing in solution other products of the reaction.

Some of the disadvantages of such a process are that difiiculty isencountered in removing other acid radicals from the chromic acid, andthat the alkali originally combined as 'chromate or dichromate appearsas a salt such as sodium sulphate. This salt is not of much value and isnot suitable for use in preparing new quantities of sodium chromate.

I am aware that in the prior art, an electrolytic cell containing adiaphragm has been used for the purpose of converting alkali chromatesto alkali dichromates. This process is limited to the preparation ofdichromates and employs a diaphragm rendering the construction of thecell more complex and greatly increasing the electrical resistanceencountered. An object of the present invention is to produce byelectrolysis from water soluble chromates, dichromates or mixtures ofchromic acid and salts of chromic acid; a solution floating in the anodechamber of the cell which solution has a substantially greater ratio ofchromic acid to base than has the feed solution, and a floating solutionin the cathode chamber of the cell which has a substantially greaterratio of base to the chromic acid than has the. feed solution, the abovedescribed result to be attained without the use of a diaphragm.

A further object of the invention is to produce chromic acid free fromother acid radicals and from alkali salts.

A further object is toproduce chromic acid electrolytically. Anadditional object is to revivify used chromium plating solutions.

Another object of the invention is to provide an apparatus or multiplechamber cell adapted to function with three different solutions andWherein means are provided for maintaining these solutions at theirdesired compositions.

The invention has for a further object to provide a multiple chambercell wherein the planes of separation of the solutions may be observed.

With the foregoing and other objects in view, the invention will be morefully described hereinafter, and will be more particularly pointed out 5in the claims appended hereto.

In the drawing, wherein like symbols refer to like or correspondingparts throughout the several views,

Figure l is a vertical transverse section taken 10 through a cellconstructed according to the present invention substantially onthe linel| of Figure 2 and Figure 2 is a horizontal section taken through thelower end of the cell on the line 2-2 01' Figure 1. v p

Referring now to the drawing which shows one form of the improvedapparatus of this invention by means of which the improved process ofthis invention may be practiced, l0 designates the body of the cellwhich may be of substantially rectangular form and having appreciabledepth and which may be made of glass, stoneware or other suitablematerial for containing chromic acid and caustic alkali. The cell isdivided longitudinally by an intermediate partition I I extending fromthe upper end of the cell down to about two-thirds or three-quarters ofthe cell depth and dividingthe cell into opposed chambers l2 and I3 inthe upper part thereof. The chambers I2 30.

and I3 intercom'municate at their lower ends through a bottom chamberId. The cell In is provided in one or both sides with windows l5 whichmay comprise panes of glass or other transparent material sealed overopenings in the walls of the cell and which are disposed above thehorizontal plane of the lower end of the partition ll. Of course thesewindows I 5 may be located in other walls of the cell or disposed at anyother points of vantage for carrying out the 40 purpose. The bottomchamber I4 of the cell is open to a feed pipe l6 which may open at anysuitable point into this chamber for supplying thereto feed solution.

The cell body in is provided at its upper end with overflow pipes l1 andI8 which communicate at their inner ends with the upper chambers I2 Iand i3 respectively and at a desired height in the upper chamber tomaintain a definite surface level in said chambers.

The solution supply pipes l9 and 20 lead into the tops of the chambersi2 and i3 respectively for introducing thereto water or for introducinginto each upper chamber a diluted solution of its product andcommunicating outside the cell with do any suitable source of supply.Each of the solution supply pipes l9 and 20 extends into its respectivechamber not lower than the plane of the electrode and is provided at itslower end with a returned portion 2| to prevent disturbance whenintroducing the solutions. In each of the chambers l2 and I3 issuspended by any suitable means an electrode 22 and 23. The electrode 22may be the anode and 23 the cathode of the cell, the annode 22 beingpreferably of lead or antimonial lead and the cathode 23 being of iron,steel or copper. The electrodes 22 and 23 may be connected to anysuitable source of direct current for operating the cell.

The process or method of the present invention, set forth and describedin connection with the above specifically described embodiment of thecell of this invention, is as follows:

In the following description, sodium chromate is used as an example butit is to be understood that other alkali salts of chromic acid or spentchromic acid solutions may be used. A sodium chromate solution havingfor example a specific gravity of 1.400 is introduced into the bottomchamber l4 through the feed pipe IE to a surface level above thehorizontal plane of the lower edge of the partition H and within theline of vision of the windows l5. When making pure chromic acid, acidradicals other than the chromate radical should be excluded from thissolution. In the anode chamber I2 is floated a solution of chromic acidhaving a specific gravity materially less than the feed solution, forexample 1.200. The solution supply pipe 19 is used to accomplish thisfloating in order to maintain the plane of separation between the anodesolution and the feed solution. Similarly a caustic soda solution ofapproximately the same specific gravity is floated in the cathodechamber being introduced through the solutionsupply pipe 20.

The cell is now placed in operation by connecting the electrodes 22 and23 to the proper poles of a source of direct current. Passage of currentthrough the cell is necessary for the maintenance of the planes ofseparation.

Stoppage of current supply is followed by diffusion across and eventualdisappearance of both planes of separation. In cells constructed ofmetal the supply voltage should not be high enough to cause passage ofcurrent through the walls of the cell itself. About 4.5 volts canusually be used with good results. Warming of the cell is beneficial indecreasing the resistance to be overcome. In operation of the cell,passage of current causes chromic acid together with a small quantity ofbasic ions to accumulate in the layer floating in the anode chamber andcaustic soda together with unused sodium chromate in the floating layerin the cathode chamber. As electrolysis proceeds the plane of separationat the top of the feed solution may be observed through the windows l5to move downward in both-chambers. This movement is to be balanced by'the introduction of additional feed solution through the feed pipe IS.

The solution in the anode chamber increases in volume and density duringthe operation of the cell. The increase in volume iskept down by meansof the overflow pipe 11. The density of the solution is maintained atthe standard by the introduction of water or diluted overflow throughthe solution supply pipe l9. The overflow consists in the chromic acidsolution produced by the cell.

Likewise in the cathode chamber Hi the volume is controlled by theoverflow pipe l8 and the requisite density maintained by dilutionthrough the solution supply pipe 20. In this chamber the overflowsolution consists of caustic soda and sodium chromate.

The chromic acid produced as described may not be satisfactory for usein a plating bath. Therefore I call this solution crude chromic acid andit may be refined by passing through a second cell identical with thefirst. This crude solution may be evaporated to reduce the volume andincrease the specific gravity and then fed through the feed pipe l6 ofthe second cell. The floating layers are maintained as in operation ofthe first cell. The acid overflow from the tube I! will now be found tobe a nearly pure chromic acid solution suitable for use in chromiumplating baths. Overflow from the cathode will be comparatively richer insodium chromate and poorer in caustic soda than is the case of a cellfed with sodium chromate solution. This last overflow may be re workedin a cell using a sodium chromate feed to produce crude chromic acid andstrong caustic solution.

To revive chromium plating baths that have become inoperative because ofan accumulation of basic ions, the solution to be revived is fed througha feed pipe l6 and the cell operated as described for reflningcrudechromic acid.

Basic ions present in used plating solutions accumulate in the floatinglayer in the cathode chamber and any insoluble hydroxides or oxides willappear as a suspension or sludge. Working of the cell is benefited bymaintaining a fairly rapid flow of solution through the solution supplytube 20 and out of overflow tube l8. In this way the insolublehydroxides or oxides are washed out of the cell. Clear red solution willbe discharged from the overflow tube I1 as distinguished from the usualdark color of used plating solutions. The product discharged fromoverflow tube I! when made up to its original volume and CrO; contentwill be found to be restored in its plating efficiency.

In some methods of cell construction it is not practical to provide twoobservation windows as shown. Normal operation of such a cell can beobtained with the use of only one observation window if the followingconditions are fulfilled. Overflow tubes l1 and I8 should be at the samelevel and the overflows from such tubes be maintained at the same ornearly the same specific gravity, variable for example between thelimits 1.190 and 1.210. It will be evident that if the specificgravities of the solutions in each chamber are equal then the planes ofseparation will be at the same level. If the cell is constructed oftransparent material windows are not necessary.

A convenient way of assuring separation as before described is toprovide a flow of solution through each upper compartment such that therise of specific gravity is not disturbing, for example the solutionfeed may have a specific gravity of 1.190 and the overflow a specificgravity of 1.200.

It must benoted that in operation of the cell the plane of separation ofthe feed solution and upper chamber solutions should be kept at a pointbelow the level of the electrodes 22 and 23 and above the plane of thelower edge of the partition II. The cell will not operate if the planeof separation rises above the electrodes or yet if the two upper chambersolutions intermix below the partition II. The windows l5 are sopositioned that when the planes of separation are kept within their lineof vision, those adverse conditions are prevented.

The above description refers to a simple cell having only two upperchambers. The cell may have two or more upper chambers, in each of whichis suspended an electrode and if composed of more than two upperchambers, each alternate chamber will have suspended in it an anode andthe intervening chamber will have suspended in it a cathode. 7

It will be obvious that various changes and modifications may be made inthe apparatus and process above described without departing from thespirit of this invention and limited only by the scope of the followingclaims.

I claim:

1. An apparatus for an electrolytic process em ploying three solutions,one of greater density than the other two, consisting of a vesselprovided with an inpermeable partition at its upper portion formingupper anode and cathode cham bers open at their lower ends, a commonchamber beneath said partition and communicating with said upperchambers, the densest solution in said common chamber and extendingpartly into the upper chambers, the light solutions floated on saiddense solution one in each of the upper chambers and separated by thedense solution, means for observing the plane, of separation between thesolutions at a point above the lower edge of the partition, an electrodein each of the upper chambers, feeding means for the common chamber, andfeeding and overflow means for each of the upper chambers.

2. An apparatus for theelectrolytic separation of a solution of analkali salt of chromic acid or of a solution containing chromic acid andbasic constituents into two parts, said apparatus consisting of a vesselcontaining an anode chamber, a chromic acid solution of relatively lowdensity in said anode chamber, a cathode chamber, an alkali solution ofrelatively low density in said cathode chamber, said chamberscommunicating at their lower ends to provide a common chamber, a feedsolution' of relatively high density in said common chamber and on whichthe anode and cathode solutions are floated, said feed solution fillingthe common chamber and extending partly into the electrode chambers toseparate relatively high density in said common chamber, said anode andcathode chambers opening at their lower ends into said common chamber,electrodes in the anode and cathode chambers, and means in the vesselfor visual observation of the line of separation between the electrodechamber solutions and the common chamber solution. t

4. An electrolytic cell comprising a vessel, a partition dividing theupper part of the vessel into two chambers communicating at their lowerends to provide a common chamber, a relatively dense teed solution insaid conmion chamber,

.relatively light product solutions floated upon said feed solution insaid upper chambers, a window in the vessel above the lower edge of thepartition, electrodes suspended one in each of the said upper chambers,a feed solution pipe communicating with the common chamber, overflowpipes leading from the vessel near the upper edges of the upperchambers, and a solution teed pipe extending down into each of the upperchambers.

5. An electrolytic cell comprising a vessel, a partition dividing theupper part of the cell into chambers communicating at their lower endsto provide a common chamber, a relatively dense feed solution in saidcommon chamber, relatively light product solutions floated upon saidfeed solution in said upper chambers, means for Ohserving the plane ofseparation between a light and the dense solution at a point above thelower edge of the partition, electrodes suspended in

